The present invention relates to an electrosurgical apparatus of the type comprising a high frequency power generator, a high frequency transformer, a handpiece for mounting an electrode, and a connection wire between the generator and the handpiece. Such apparatuses are used in particular by surgeons and dentists to perform incisions and also to stop bleeding.
In electrosurgical equipment of the type in question, a high frequency current is conducted through a relatively thin, wire-shaped electrode which remains cold because of good conductivity, but when the electrode is moved through tissue, heat is generated, a.o. because of the electrical resistance of the tissue. This causes a molecular dissolution of the tissue cells, since the generated heat causes both intra- and extracellular generations of steam which bursts the tissue. How deep in the tissue this dissolution takes place, depends on the intensity of the current and on the speed at which the electrode is moved through the tissue.
An important advantage of such an electrode dissection instead of using a scalpel resides in the fact that the operating area is free from bleeding. This is an important advantage in connection with the taking of impressions for tooth crowns and bridges, as an impression may be taken immediately after the necessary incisions have been made. Electro-surgery is also well suited for preposthetic surgery and paradontal surgery.
Electrosurgical equipment has, however, not yet gained the extended use which the important advantages should justify. This is due to several substantial drawbacks. Thus, it is difficult to use the apparatus or its electrode in a correct manner. If the electrode is kept still, the destruction of tissue will become very extensive and may result in periostitis, bone tissue necrosis and sequestrian of bone tissue. With too high electrode voltage or, if the electrode is moved too slowly through the tissue, permanent and very painful damages may be done to bone tissue and also to tooth substances. For the time being, the use of electrosurgery is therefore deprecated, when the electrode may approach or get in contact with bone tissue, and this is a substantial and unfortunate limitation of the possibilities of use.
Another considerable disadvantage consists in the fact that it is difficult to control the power output of the apparatus in a sufficiently effective and precise way. When cutting with the electrode of the apparatus, the necessary or desired output power varies with the depth of the cut, with the impedance provided by the patient, and with numerous other factors relating to the shape of the electrode, the environment and the particular operating procedure. This has had the effect that prior electrosurgical apparatuses have been provided with several adjustments for manually presetting the power output, such as a pre-adjustment for each electrode belonging to the apparatus, whereby a desired output power may be selected, usually by adjusting the output voltage.
U.S. Pat. No. 4,092,986 discloses an electrosurgical unit of the type contemplated which includes an inverse feed-back circuit to maintain the output voltage level from the unit at a substantially constant value independent of the load.
However, when the output voltage is applied to a patient by means of the electrode, the patient will act as a load which causes that the output voltage level decreases. The magnitude of the load or of the voltage decrease depends on the electrical resistance of the tissue in which the cut is being made and of the instantaneous depth of the cut. Thus, the load resistance decreases as a function of the cutting depth and, moreover, the load resistance depends on the type of tissue which the electrode is encountering. Accordingly, a continuous regulation is necessary as the load is changing, and the voltage delivered by the electrode should not be constant.
Another commercially available electrosurgical apparatus comprises a double feed-back circuit and a data analyser to maintain an electrode output voltage level which is automatically and continuously adapted to the electrical tissue resistance and to the variations in the depth of the cut so that the power delivered by the cutting electrode currently is proportional with the cutting depth. However, the electronics of the apparatus introduce time constants and, accordingly, it will be difficult to obtain a sufficient quickness in the adaptation or regulation.